How about having someone other than the Earl of Leicester go in to help the Netherlands, someone who doesn't rankle them, keeps public support, and enables the Dutch revolt to go better.
Actualy Leicester's incompetence made the Dutch provinces band together again and pull together and basicly beat the Spanish back again. So, I would let him stay.
After this loss of its armada, Spain has to declare bankruptcy again, but can't resupply with gold and silver from the New World. Combine this with the
Battle of Gibraltar in 1607, and, through pure luck, the survival of Admiral van Keemskerk, with Prince Maurice not halting the march to Dunkirk (or going elsewhere?), and perhaps the Spanish can be forced into a treaty rather than truce, yielding the Southern Netherlands to the Dutch in their entirety.
No, the Spanish would not accept that. They would not make peace with a protestant nation. Besides that the Spanish position in the Netherlands was still way too strong to be beaten back so easily, even with a deteriorating financial situation. Don;t forget, Spain went backrupt OTL too and the Netherlands wasn't able to defeat the Spanish. Besides that the Dutch financial situation wasn't that great either. That only happened during the 12 year truce, when the Dutch basicly went colonizing around the world and trading around Europe (which was way more profitable than the early colonisation attempts).
What would you figure as a timeline for this Netherlands, if you had to guess?
So far, we have:
1588-89: Spanish Armada/English Armada, ending in decisive English victory, dampening Spain's ability to project power
If I would make a guess, this would happen:
The Netherlands is in a slightly better position than OTL and manages to conquer Den Bosch and Venlo in 1598. Besides that, they manage to supply Ostend (ok, I will admit that it is unlikely, but I can see it happen), the seige continues, but Maurice manages some relieve for the city and the town can be supplied from the sea. Because of the worse financial situation Spinola is not able to attack Twente and the achterhoek in 1605, meaning it remains Dutch. Stil both sides cannot gain an advantage and like OTL a truce is created, since the Spainsh refuse to make peace.
Differences between OTL and ATL:
Ostend remains Dutch (and protestant)
Various towns in the east remain Dutch
Den Bosch and Venlo are Dutch
When the truce end the Dutch can focus solely on the south, instead of OTL, when the Netherlands had to focus on the east too. This means that the Dutch republic manages to increase their territory and conquers Antwerp and Bruges (and some area around it). France will still be able to conquer Dunkirk and Artois. At the end of the war the southern Netherlands will be smaller, but none of the Walloon areas will be Dutch. There was hardly any support for the Dutch revolt in those areas (and there never realy had been any, except some parts of French Flanders). I also don't think the Dutch are able to conquer all of the Dutch speaking areas, Brussels is simply too far away. So you get a Spanish Netherlands that is smaller and barely has a coastal access (just Nieuwpoort), so making it less interesting for, well anyone besides France and the Netherlands.
Since the Netherlands now controls Ostend, a major Flemish city (and protestant, thus important) and Antwerp (a major port thus important), it probably want to extend its borders in future wars to protect those cities. Because of this, I believe the Dutch Republic will want to get more territory of the Southern Netherlands in future wars, somehow. I could easily see some fortified cities, like Nieuwpoort or Ghent ending up Dutch to protect Ostend and Antwerp from possible French agression.
The Walloon areas, no. They aren't important enough for the Dutch. They are Catholic, speak a different language and are strategivcally less important. I could see a line of barriere cities in Wallonia though. Towns in which the Dutch republic pays for the defenses, although they remain officialy outside of the republic's control.
I know you like the idea of a united Netherlands, including the Walloons (I do too, it would be very interesting in the 17th-18th century), but I simply don't think it can happen. Mind you, this Netherlands, which includes Ostend, Bruges and Antwerp and a relatively large part of the Flemish coast will be a stronger naval power than it was OTL. Antwerp would be a major boon for the Dutch and Ostend is far from worthless. I could see both developing rather well. And a Dutch Antwerp is probably good news for the province of Zeeland too (Ships for Antwerp must go through Zeeland after all). Only Amsterdam is probably worse off (although it will still be incredibly wealthy, like OTL).
Other interesting ideas will be Dutch internal politics, which will probably differ a lot from OTL. I am uncertain if you are interested in my speculation though, if you don't know anything about internal Dutch politics from the 17th century.